A killer disease is decimating fish stocks in the Zambezi River Valley, threatening the food security and livelihood of hundreds of thousands of people in an area shared by seven southern African countries, a United Nations agency said Tuesday.

The most affected country is Zambia, where two thirds of the Zambezi River Basin lies, the Rome-based Food an Agriculture Organization (FAO) said.

Over 2,000 villages and some 700,000 people are at risk of food insecurity because fish is not only a source of revenue in many rural districts but is also the cheapest available source of protein, FAO said.

The disease, known as Epizootic Ulcerative Syndrome, or EUS, is caused by the fungus Aphanomyces invadans, which forms ugly lesions on fish and has a high rate of mortality.

Fish infected with EUS do not normally pose health hazards to humans, although the deep ulcerations and tissue decay characteristic of the disease could harbour secondary, more threatening pathogens.

EUS-affected fish is un-marketable, causing severe economic loss to fishers and fish farmers.

"If not properly contained there is the risk of the disease spreading to other countries surrounding the Zambezi River as well as river systems in the region," FAO Senior Fishery Resources Officer, Rohana Subasinghe said.

The 1.39 million-square-kilometre Zambezi River Basin is home to some 32 million people, of whom 80 per cent are dependent on agriculture or fishing and fish farming.

The Zambezi River Basin also includes Angola, Botswana, Malawi, Mozambique, Namibia, and Zimbabwe.

Control of EUS in natural waters such as rivers is "impossible," FAO said.

However, control is relatively simpler in fish farming operations where a number of simple biosecurity measures can minimize or prevent its spread.

They include preventing possible carriers or vectors getting into water bodies or fish ponds, removing dead fish and improving water quality.